This invention relates generally to an electrophotographic printing machine, and more particularly concerns an apparatus for regulating the developability of a development system employed therein.
In the process of electrophotographic printing, a charged photoconductive member is exposed to a light image of an original document being reproduced. The irradiated areas of the photoconductive surface are discharged to record thereon an electrostatic latent image corresponding to the original document. A development system moves a developer mix of carrier granules and toner particles into contact with the photoconductive surface. The toner particles are attracted electrostatically from the carrier granules to the latent image forming a toner powder image thereon. Thereafter, the toner powder image is transferred to a sheet of support material. After transferring the toner powder image to the sheet of support material, a fusing device permanently affixes the toner powder image thereto.
It is evident that in a printing machine of this type, toner particles are depleted from the developer mixture. As the concentration of toner particles decreases, the density of the resultant copy degrades. In order to maintain the copies being reproduced at a specified minimum density, it is ncessary to regulate the concentration of toner particles in the developer mixture. This is the function of the developability regulating apparatus.
Developability, as it pertains to an electrophotographic printing machine is the ability of the developer mixture to develop the image with at least a minimum specified density. The regulating apparatus adjusts the characteristics of the developer mixture to achieve the foregoing.
Primarily, developability is related to the concentration of toner particles in the developer mixture, i.e., the percentage of toner particles relative to carrier granules therein. Other factors effect developability such as temperature and humidity conditions as well as the physical parameters of the development system such as spacing, electrical bias, mass flow rate, and the magnetic field pattern, amongst others. Numerous systems have been developed for controlling the concentration of toner particles within a developer mixture so as to maintain the resultant image density at least at a minimum value. For example, U.S. Pat. No. 3,635,373 issued to Kuhl et al. in 1972 discloses a system employing two parallel spaced conductive plates through which the developer mixture flows. The plates are connected to a circuit wherein each is electrically charged alternately for equal periods of a time to attract and repel toner particles. A light source is located on one side of the two plates with a photocell being located at the other side to sense the illumination intensity transmitted therethrough. The photocell develops an electrical signal which is processed to form an error signal. The error signal controls the dispensing of toner particles into the developer mix. Another patent is U.S. Pat. No. 3,757,999 issued to Maksymiak in 1973. This patent teaches the use of spaced conductive plates alternately electrically charged between which the developer mix flows. A light source is positioned on one side of the two plates and a photocell on the other side thereof. In this way, the intensity of the light rays passing therethrough is detected. This system also provides a measurement of the toner particles concentration within the developer mix. Still another patent disclosing a system of this type is U.S. Pat. No. 3,376,854 issued to Kamola in 1968. Kamola describes a toner concentration control system wherein two parallel spaced conductive plates define a channel through which developer mix passes. One plate has a pattern thereon which is held to an electrical potential to attract the toner particles from the developer mixture. A light source and photocell are positioned with the plates interposed therebetween. Another photocell is arranged as a leg of a bridge circuit which includes the first photocell. In this way, an unbalance in the bridge causes toner particles to be dispensed to the developer mixture.
It has been found that devices hereinbefore employed frequently have significant signal fluctuations due to the developer flow between the plates. This significantly reduces the control accuracy of the system.
Accordingly, it is a primary object of the present invention to improve the control accuracy of a developability regulating apparatus.